Intensity modulated radiotherapy (commonly referred to as IMRT) is a generic term for a number of radiotherapy techniques that, essentially, vary the beam intensity that is directed at the patient. That variation can be spatial, temporal, or both.
In radiation therapy the terms dose, fluence and intensity are sometimes used interchangeably and confusingly. For the purposes of this description and this application these terms are used as follows. Fluence is the number of photons or x-rays that crosses a unit of area perpendicular to a radiation beam. Fluence rate is the fluence per unit time. Intensity is the energy that crosses a unit area per unit time. Fluence and intensity are independent of what occurs in a patient, and more specifically are not dose. Dose is the amount of energy absorbed by tissue by virtue of radiation impacting the tissue. Radiation dose is measured in units of gray (Gy), where each Gy corresponds to a fixed amount of energy absorbed in a unit mass of tissue (e.g., 1 joule/kg). Dose is not the same as fluence, but increases/decreases as fluence increases/decreases.
In radiation therapy delivery, the beam aperture is commonly set by a multi-leaf collimator (MLC). One such method of using the MLC is to create one or more patterns that shape the radiation. A single shape that matches a target is commonly referred to as a conformal delivery. For more complicated dose distributions IMRT can be utilized. In IMRT, rather than having the MLC shape the incident radiation to match a certain outline, the MLC is instead used to create an array of beam shapes that create a desired intensity modulation and desired 3D dose distribution.
FIG. 1 illustrates an isometric view of a conventional shaping-MLC 31 (such as those used on Varian radiation therapy systems) passing a beam to a target in a patient. Two banks 33, of opposing leaves, where each leaf 37 may be positioned continuously across the radiation field. The two banks of leaves are positioned so as to collimate the beam 30 in the desired shape. Each leaf 37 typically may travel beyond the midpoint of the collimator in order to provide flexibility when achieving the desired collimation. The configuration illustrates fully open (41), partially open (43) and closed (45) leaf states.
In an example of radiation therapy, each gantry angle has one beam associated with that particular gantry angle, which beam 30 is then collimated into multiple shapes by an MLC. Treatment beam 30 passes through the shaped aperture 47 formed by the leaves 37. The resulting collimated beam continues onto a target 14 within the patient 38. FIG. 1 also illustrates how the treatment beam may be visualized or conceptualized as many different beamlets 49. Leaves 37 of conventional shaping-MLC 31 are moved into various positions to achieve desired shapes or apertures for specified periods of time to achieve fluence map 51 for that particular beam. Modulation of the conceptualized beamlets occurs by sequentially and monotonically moving the leaves into desired positions to achieve desired shapes or appertures such that the time a conceptualized beamlet is exposed controls the intensity of that beamlet. Monotonic, as used in this application and related to radiation therapy, means an ordered sequence of apertures where the sequence is dictated by a continuum from one aperture to a subsequent aperture or where individual leaves increment in one direction during a given series of apertures. In other words, a sequence of apertures would be dictated by mechanical limitations of the MLC, not so much by what may achieve the more optimal treatment delivery; a sequence would go from aperture 1, then 2 then 3 and so on, and not from 1 to 3 then to 5 then back to 2. Rather than use a single conformal shape, the MLC delivers a sequence of shapes. The net amount of radiation received at any given gantry position is based upon the extent to which the different shapes permit the passage or blockage of radiation. As seen in FIG. 1, the shape of MLC 31 shown does not directly correspond to the beamlet intensities of the fluence map 51. As will be appreciated, the depicted fluence map shows the accumulation of intensities for multiple shapes the MLC has taken for that particular gantry angle. A common limitation of the conventional shaping MLC is that the leaves defining the shapes move relatively slowly. Using large numbers of shapes, or shapes that require large leaf motions, can result in longer patient treatments. Likewise, the speed of the leaves can limit the ability of conventional shaping-MLC's to deliver time-sensitive treatments, such as utilizing synchronized motion of delivery components (e.g., gantry, couch, x-ray energy etc.).
A conventional binary MLC 61 is shown in FIG. 2. The binary MLC 61 has a plurality of leaves 63 arranged in two banks 65, 67. Each bank of leaves is used to form a treatment slice by positioning the leaf in a closed position or open position with respect to the beam. As shown in FIG. 2, the leaves may work in concert to be both open (A), both closed (B) or where only one leaf is open/closed (C).
Binary MLCs are used in TomoTherapy's Hi-Art® radiation therapy system and the North American Scientific treatment system. In the conventional binary-MLC treatment system the patient is moved past the rotating radiation source to deliver a helical treatment to the patient using the dual bank binary collimator. Alternatively, the patient is indexed for treatment of another subsequent two slices by the dual bank binary collimator, as is done by the North American Scientific system. Leaves of the dual bank binary collimator move with sufficient speed such that leaf sequencing or positioning will not be significantly influenced by any previous or future positions (open or closed for a binary collimator) of any individual leaf. Stated another way, leaf speed is sufficient such that the mechanics of the MLC do not unduly influence the determination of leaf position at any given time for the delivery of a radiation therapy treatment or fraction. Thus, and in contrast to the conventional shaping-MLC, each leaf defines a beamlet that does not require conceptualization by the planning software, i.e., the amount of time a leaf is open directly controls the intensity for that beamlet.
For both conventional MLCs (shaping and binary), each beamlet has a fluence and all the fluences combined form a fluence map for the beam. The fluence maps for each gantry angle or for all the beams are combined and optimized into the treatment plan. The example of the conventional shaping MLC has been provided to illustrate the underlying concepts of volumetric intensity modulation using the shaping MLC and that of the binary MLC to illustrate the underlying concepts of direct intensity modulation at discrete gantry angles. More complicated treatment plans and delivery can include gantry motion, couch motion, varying gantry speed, varying MU, etc. in order to provide more sophisticated and theoretically better dose conformation in less time per fraction. It is the treatment plan, via delivery software, that governs the operation of the treatment delivery device. The physical capabilities of the delivery system (gantry, linear accelerator, MLC, couch, etc.) limits or constrains the treatment planning software in the type of plan it can create and optimize for delivery by the delivery system.
Treatment planning systems and software (collectively referred to as planning system) are not the focus of this application, but, and as will be appreciated, are integral for treating a patient with radiation. Radiation therapy treatments are governed by a treatment plan, typically generated by a physician or physicist (alone or collectively a “planner”) using the planning system. The planner will typically use a diagnostic 3D image (typically CT, although combinations of any PET, CT, MR maybe used) and define or contour the target structure and any nearby critical structures or organs at risk (OAR). The planner then determines the amounts of radiation for delivery to the target structure and the amount of radiation that will be allowed to the OAR. The treatment planning software, using inverse planning and the physical capabilities of the delivery device will generate a treatment plan. The planner then evaluates the plan to determine if it meets the clinical objectives, and if so will approve the plan for delivery to the patient. Delivery of the plan occurs in multiple treatment sessions or fractions.
Conventional MLCs and the treatment paradigms resulting from them have provided steadily advancing and more sophisticated conformal radiation therapy treatments. However, there remains a need for more advanced shaping and modulation of the therapeutic beams, thereby enabling treatment planning software to develop and enable delivery of even more sophisticated plans. As seen by the above summary of radiation therapy techniques, one key component for delivery of radiation therapy is the collimator. While multi-leaf collimators exist, the speed and control of an individual leaf or group of leaves is insufficient to achieve more advanced simultaneous shaping and modulating beam patterns. What is needed are improved multileaf collimator designs, responsive enough to meet the speed and position control requirements of more advanced radiation treatment plans, thereby enabling new treatment paradigms.